The present invention relates to the general field of turbine engines having gearing driven in rotation by means of a single turbine. It is thus equally applicable to airplane turboprops having one or two propellers comprising a single blade set or two unducted contrarotating blade sets, and to airplane turbojets having one or two fans comprising one blade set or two ducted contrarotating blade sets. The invention relates more particularly to an epicyclic gear device used between the shaft of the turbine rotor and the drive shaft(s) for driving rotation of the blade set(s) that is/are driven by the turbine.
In known manner, an airplane turboprop with two propellers has two contrarotating unducted blade sets. In certain two-propeller turboprop architectures, the two blade sets are driven in rotation directly by a power turbine having two contrarotating rotors. In other architectures to which the present invention relates more particularly, the blade sets are driven by a single rotor of the power turbine. By way of example, reference may be made to Document FR 2 979 121, which describes an embodiment of such an architecture.
The contrarotating propellers of such a turboprop may be driven in rotation directly or indirectly via a mechanical transmission device forming a speed reducing gearbox and including an epicyclic gear train. In general, the epicyclic gear train comprises a set of rolling elements with straight teeth including a sunwheel that is centered on the longitudinal axis of the turboprop and that is connected upstream to the rotor of the power turbine in order to be driven thereby. Downstream, the sunwheel acts via the ring and the planet carrier to output its rotary motion at different speed and torque to the two blade sets of the turboprop.
Given the mechanical environment in which the turboprop exists, misalignments between the axes of the shafts connected to the various rolling elements of the epicyclic gear have an impact directly on their teeth. By acting as bearing points seeking to bring misaligned shafts into alignment, the teeth are either subjected to premature wear, which is extremely deleterious to the lifetime of the gear as a whole, or else they require increased thicknesses of material of a kind that leads to highly penalizing constraints on integration when developing an on-board device of weight and size that must necessarily be minimized.
In order to reduce significantly the concentration of stresses in the straight teeth of such epicyclic gearing, it is known to create a difference between the bending stiffnesses of the inlet and outlet shafts of the gearing, thereby having the effect of reducing the force needed to bring the ends of the shafts into alignment. Any misalignments between the shafts are thus compensated by the flexibilities of the shafts, and they are then taken up at the contacts between the straight teeth of the rolling elements of the epicyclic gear by relative sliding between them.
Nevertheless, that solution still presents numerous drawbacks. In addition to such straight-tooth epicyclic gearing having power density that is rather low, it generates a large amount of noise, since force transmission is interrupted by jolts that occur on transition from one tooth to another, thereby generating metallic clicks. Furthermore, the repeated mechanical impacts imposed on the teeth reduce their lifetime.
There therefore exists an unsatisfied need for an epicyclic gear device for a turbine engine with gearing in which the power density is significantly increased.